1. Field of the Invention
The present invention relates to a photothermographic material, and particularly, to a photothermographic material containing a novel chemical sensitizer. Further, the present invention relates to a photothermographic material that is produced using a silver halide emulsion having a high silver iodide content, and exhibits considerably improved sensitivity, has low Dmin and high Dmax, and excellent storability before being subjected to a developing treatment, and further has excellent image storability after being subjected to the developing treatment.
2. Description of the Related Art
In recent years, photographic development processing conducted in a dry state is strongly desired in the fields of medical and printing plate making from the standpoints of environmental protection and space saving. In these fields, digitalization is progressing, and accordingly a system in which image information is taken in a computer, stored, optionally processed, transmitted, outputted to a photosensitive material by a laser image setter or a laser imager, and developed to form an image is rapidly spreading.
As for the photosensitive materials, capability of recording by laser exposure of high illuminance and forming a clear black image having high resolution and sharpness is required. As for digital imaging recording materials, various types of hard copy systems utilizing a pigment or a dye, such as an ink-jet printer and an electronic photographic system, are distributed as an ordinary image-forming system. However, none of the hard copy systems are satisfactory with regard to image quality (sharpness, graininess, gradation, and color tone) used in the medical field for diagnosis and recording speed (sensitivity) for the purpose of replacing conventional wet-development-type silver salt film for medical use.
On the other hand, thermally developable image-forming systems utilizing an organic silver salt are described in, for example, U.S. Pat. Nos. 3,152,904 and 3,457,075 and D. H. Klosterboer, “Thermally Processed Silver Systems” (see Imaging Processes and Materials, Neblette, 8th Ed. compiled by J. Sturge, V. Walworth and A. Shepp, Chap. 9, page 279, 1989). In particular, the photothermographic material comprises a photosensitive layer in which a photosensitive silver halide, a reducing agent, a reducible silver salt (e.g., organic silver salt), and optionally, a toning agent for controlling silver color tone are ordinarily dispersed in a binder matrix.
When the photothermographic material is heated at a high temperature (e.g., 80° C. or higher) after being imagewise exposed, a monochromatic black silver image is produced by a redox reaction between the silver halide or the reducible silver salt (functioning as an oxidizing agent) and the reducing agent. The redox reaction is accelerated by a catalytic action of a latent image of the silver halide formed by such exposure. As a result, the monochromatic silver images are formed at exposed areas of the material. Such photothermographic materials are disclosed not only in U.S. Pat. No. 2,910,377 and Japanese Patent Application Publication (JP-B) No. 43-4924, but also in many references other than those described above. Thus, Fuji Medical Dry Imager FM-DP L was launched on the market for practical use as an image-forming system for medical use utilizing the photothermographic material.
Since such an image forming system utilizing the organic silver salt includes no fixing step, it has a problem in image storability after being subjected to a developing treatment, particularly a problem of deteriorated print-out upon light exposure. As a method for improving such a printout problem, a method which utilizes silver iodide obtained by converting an organic silver salt is disclosed (see, e.g., U.S. Pat. No. 6,143,488 and EP-A 0922995). However, each method of converting the organic silver salt by iodine as disclosed in these patents was incapable of obtaining sufficient sensitivity, whereupon it was difficult to construct a practical system. Other sensitive materials utilizing silver iodide which are described in some references cited in the specification of patents (see, e.g., WO97/48014, WO97/48015, U.S. Pat. No. 6,165,705, Japanese Patent Application Laid-Open (JP-A) No. 8-297345, and Japanese Patent No. 2785129) have not attained sufficient sensitivity and fogging levels, and these materials are not practical to be used as materials sensitive to laser exposure.
As a measure for increasing sensitivity of a silver iodide photographic emulsion, it has been known in literatures that sensitization is performed by utilizing halogen receptors such as sodium nitrite, pyrogallol and hydroquinone, immersion in a silver nitrate aqueous solution, or sulfur sensitization at pAg 7.5 (see, e.g., P. B. Gilman, Photographic Science and Engineering, Vol. 18(5), page 475 (1974), W. L. Gardner, ibid. Vol. 18(5), page 475 (1974), or T. H. James, ibid. Vol. 5, page 216 (1961)). However, a sensitizing effect exerted by these halogen receptors in photothermographic materials is known to be very small and thus unsatisfactory. Accordingly, development of a technique which is capable of substantially enhancing sensitivity of the photothermographic material having a high silver iodide content has been desired.
On the other hand, a technique has been disclosed in which, by using a compound having an adsorptive group to a silver halide and a reducing group or a precursor thereof, sensitivity of a silver iodide emulsion having a low silver iodide content can be enhanced for use in a color negative emulsion or an X-ray emulsion usable for a liquid developing treatment (see, e.g., JP-A No. 8-272024).
However, in a case of a silver halide photosensitive material used in a liquid developing treatment, a silver image is ordinarily formed by reducing silver halide by means of a developing agent (reducing agent) contained in the developing liquid, or a color image is formed by making use of an oxidized form of the developing agent to be generated as a by-product of a fundamental reaction to reduce the silver halide by the developing agent. On the other hand, in a case of the photothermographic material, the silver halide only acts to form a latent image by light exposure and the silver halide itself is not reduced by the reducing agent. Namely, it is a silver ion supplied from a reduced non-photosensitive organic silver salt. Reducing agents in the case of the photosensitive material used in the liquid developing treatment are ionic reducing agents such as hydroquinones, 3-pyrazolidones and p-phenylene diamines, while reducing agents in the case of the photothermographic material are hindered phenol derivatives which are ordinarily known as radical reacting agents.
As described above, in the photosensitive material for the liquid developing treatment and the photothermographic material, mechanisms of developing reactions (reduction reaction) are completely different from each other in the above-described cases, and hence compound systems to be used in these cases are completely different from each other. Therefore, compounds which are effective in the liquid developing treatment are not always effective as they are when applied to the photothermographic material. Applying compounds described in the foregoing JP-A No. 8-272024 to the photothermographic material has not yet been conceived, let alone applying those compounds in the photothermographic material that has a high silver iodide emulsion. Thus, it was impossible to estimate an effect thereof.